Modern oil field operators demand access to a great variety of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and the process of collecting such information is commonly referred to as “logging”. Logging can be performed by several methods including wireline logging, tubing-conveyed logging, and “logging while drilling” (LWD).
In wireline logging, a sonde is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
Tubing-conveyed logging is similar to wireline logging, but the sonde is mounted on the end of a tubing string. The rigid nature of the tubing string enables the tubing-conveyed sonde to travel where it would be difficult to send a wireline sonde, e.g., along horizontal or upwardly-inclined sections of the borehole. The tubing string can include embedded conductors in the tubing wall for transporting power and telemetry, or a wireline cable can be fed through the interior of the tubing string, or the sonde can simply store data in memory for later retrieval when the sonde returns to the surface.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being drilled. LWD enables measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations.
One of the instruments that has been employed in each of these environments is a nuclear magnetic resonance (NMR) logging tool. A brief synopsis of the operational principles behind NMR tools is provided at the end of this specification, but for the moment it is sufficient to recognize that as part of their operation, NMR tools employ an arrangement of permanent magnets to establish a strong magnetic field in some designated sensing volume. Commonly, for downhole logging, the permanent magnets are desired to create a strong magnetic field and have minimal change in elevated temperatures.
Permanent magnets are generally manufactured from a loose powder. The loose powder is ground into fine particles, which are then pressed and sintered in the presence of a magnetic field. A molding or machining process determines the final shape of the magnets. Inaccuracies in this process cause the field of the permanent magnet to deviate by as much as a degree from the desired field orientation. The deviation may stem from various possible causes including uneven cooling and machining tolerances.
One known approach to improving the performance of NMR logging tools is to increase the sensing volume to improve the signal to noise ratio. An increase of volume is inherent to a lower gradient field. This “low gradient” approach to NMR logging can be particularly sensitive to the field deviations of the permanent magnets.
It should be understood that the drawings and detailed description thereto are illustrative and do not limit the disclosure to the particular illustrated embodiments, but rather they provide the foundation for all modifications, equivalents and alternatives falling within the scope of the appended claims.